Problem: If $\angle AOC$ is a right angle and $m \angle BOC = 77^\circ$, what is $m \angle AOB$, in degrees? $77^\circ$ $O$ $A$ $C$ $B$
Solution: From the diagram, we see that $\angle AOB$ and $\angle BOC$ are complementary angles. Therefore, $m \angle AOB + m \angle BOC = 90^\circ$ Thus, $m \angle AOB = 90^\circ - m \angle BOC = 90^\circ - 77^\circ = 13^\circ$.